chore(pty/ptytest): log scanner errors during shutdown (#25485)

pty/ptytest/ptytest.go

@@ -132,6 +132,10 @@ func newExpecter(t *testing.T, r io.Reader, name string) outExpecter {
 		for s.Scan() {
 			ex.logf("%q", stripansi.Strip(s.Text()))
 		}
+		// Surface non-EOF scanner errors; otherwise they're invisible.
+		if err := s.Err(); err != nil {
+			ex.logf("log scanner stopped: %v", err)
+		}
 	}()
 
 	return ex